This invention relates generally to PIN diode photodetectors, and more particularly to a photodetector with improved responsivity.
Photodetectors are used to convert optical power into photocurrent. A typical PIN diode photodetector includes one absorption layer and two contact layers of opposite polarity with the contact layers positioned on opposite sides of the absorption layer. Light is directed into the absorption layer through one of the contact layers. The light propagates through the absorption layer which absorbs the light and converts it into photogenerated carriers in the form of pairs of positively charged holes and negatively charged electrons. The positively charged holes are attracted to and travel towards the negatively charged contact layer. The negatively charged electrons are attracted to and travel towards the positively charged contact layer. The contact layers collect the electrons and holes. These collected charge carriers constitute the photocurrent.
Bandwidth and responsivity are the most important parameters of a photodetector's performance. Bandwidth is the frequency response or speed of the photodetector measured in Hz. The bandwidth of a photodetector is determined by the transit time of the photogenerated carriers in the absorption region and the RC time constant. The transit time of the photogenerated carriers is a measure of how quickly the electrons and holes reach their respective contact layer and is determined primarily by the thickness and material parameters of the absorption layer. The thinner the absorption layer, the lower the transit time because the electrons and holes have a shorter distance to travel to reach their respective contact layer.
The RC time constant is dependent primarily on the capacitance of the photodetector which is primarily determined by the thickness of the absorption layer and the size or area of the photodetector. The thicker the absorption layer, the lower the capacitance resulting in a photodetector with improved bandwidth.
Responsivity is a measure of how efficiently the photodetector converts optical power into photocurrent. Responsivity is a measure of how much of the light which enters the photodetector is absorbed by the absorption layer and converted into photogenerated carriers. The amount of light absorbed by the absorption layer is an exponential function of the thickness of the absorption layer. Thus, a thick absorption layer absorbs more light than a thin absorption layer providing higher responsivity.
A thin absorption region is desirable for improved transit time. However, a thick absorption region is desired for lower capacitance and higher responsivity. Therefore, in typical photodetectors, there is a trade-off between bandwidth and responsivity.
One configuration employed to increase responsivity without sacrificing bandwidth is to illuminate the absorption layer from the edge of the photodetector instead of through one of the contact layers. This configuration removes the trade-off between bandwidth and responsivity since in this configuration, the transit time is dependent on the thickness of the absorption layer in a direction perpendicular to a direction of propagation of the light, but, the responsivity is dependent on the thickness of the absorption layer in the direction parallel to the direction of propagation of the light. For a high performance photodetector, the vertical distance is typically on the order of 0.3 to 2 .mu.m and the horizontal distance is typically on the order of 10 to 30 .mu.m. Therefore, for a typical edge-illuminated photodetector, the vertical thickness of approximately 0.3 to 2 .mu.m determines the transit time of the photodetector whereas the horizontal thickness of approximately 10 to 30 .mu.m determines the responsivity of the photodetector. One drawback to this technique is that, in order to couple the light efficiently into the absorption layer, the light must be focused to a small size prior to the light entering the absorption layer. Typically, the light must be focused to a diameter comparable to the thickness of the absorption layer (0.3 to 2 .mu.m). This is difficult and adds additional complexity to couple the light to the photodetector.
A second method employed to increase the responsivity of the photodetector without sacrificing bandwidth is to use a reflective layer on the bottom of the photodetector. For this method, the light enters the absorption layer through the top contact and propagates through the absorption layer. The absorption layer absorbs only a portion of the light converting that portion into photogenerated carriers. The portion of the light not converted is incident on the reflecting surface which redirects the remaining light back through the absorption layer. The remaining light propagates a second time through the absorption layer and is converted into photogenerated carriers. This method enables a higher responsivity without reducing the bandwidth. One drawback to this method is that it requires a highly reflecting surface or a multiple layer mirror underneath the photodetector. It also complicates greatly the fabrication of the photodetector particularly for systems that operate at a wavelength of 1.55 .mu.m
What is needed therefore is a photodetector which would allow for increased responsivity with minimal impact on the bandwidth but without the complexity of fabrication and without severe constraints of focusing the light.